In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
A two-piece can is made by a drawing and wall ironing process. In general, a two-piece can is made by stamping out metal discs from a metal plate. A metal “cup” is formed from the disk. The formed cups are pushed through a body-forming die comprising a plurality of annular rings, generally known as draw, redraw, and ironing rings, by a body-forming punch. The clearances between the body-forming punch and the plurality of rings become progressively smaller, so that the thickness of cup wall is reduced and the cup is elongated. This process is generally referred to as the ironing operation. It is a particularly demanding operation causing high wear on the tools and the operation is sensitive to the dimensional changes and lubrication conditions. Because of the tremendous volume of beverage cans manufactured each year, each slight improvement in the manufacturing process can result in tremendous savings.
Tools for imparting a desired shape, form, or finish to a material, such as dies, punches, and the like, must be characterized by extreme hardness, compressive strength and rigidity. This is particularly necessary when shaping metals or similar materials. Commercial material working tools for mass production must also be resistant to wear, erosion and chipping from repeated and continuous stress and abrasion. In addition, these tools should also exhibit good corrosion resistance properties in order not to be damaged by the surrounding liquid media (coolant/lubricant).
On top of these properties, others are of great importance for the punching tools. As this kind of tool is moving very rapidly, any reduction of the weight will result in huge improvements, both in term of cost and life time of the tools. Indeed, if the tool is lighter, less energy is required to run the process and the bending of the ram is reduced. This later effect results in a much better alignment of the punch within the tool-pack and less damages to the antagonist tool; the ironing dies. As a consequence, both tools (punch & dies) will be less damaged during the process due to the reduction of the bending effect.
These tools must also be made from materials which can be designed and machined to close tolerances and maintain dimensional stability over a wide range of operating conditions.
A possible way to improve wear and corrosion resistance is described in JP 3-258424 by the addition of 0.16-0.48 wt-% chromium to the binder phase and having a dispersed fine grained phase of tungsten carbide and tantalum carbide.
Another possible way to achieve both wear and corrosion resistance combined with a reduction of the material density is described in U.S. Pat. No. 5,736,658. This is linked to the use of a nickel based alloy that exhibit a better corrosion resistance and to the addition of titanium carbide, being a lighter material than tungsten carbide. However the benefit is also limited as the binder phase could be even more wear resistant than a nickel based material. In addition, the wear resistance is significantly improved by increasing the target hardness level. A hardness goal of 88 to 91 Ra (corresponding to about 1150 to 1450 HV30) is mentioned to ensure a wear resistance level approximately equivalent to the standard grades. Finally, as no cobalt is added to the binder phase, the grade is non magnetic, which could be a critical drawback for the can maker that request magnetic materials for the punch tool.
EP 1 557 230 discloses a cemented carbide body of 10-12 wt-% Co, <3 wt-% TaC, 1-5.5 wt-% NbC, 3-5 wt-% TiC and remainder WC, particularly useful for metal cutting operations requiring high wear resistance, high edge retention and high edge toughness.
However, so far the conventional cemented carbide seems to keep its position as preferred material. This is mainly medium/coarse grades with about 11 wt-% of cobalt binder or 9 wt-% of alloyed nickel based binder when non magnetic properties are required. Both grades exhibit hardness in good agreement with that mentioned above (1250 and 1375 HV30, respectively).